This invention relates to a differential amplifier utilizing a MIS (metal-insulator-semiconductor) type field effect transistor. A differential amplifier is generally used for amplifying read signals of an IC memory device or small signals of a linear circuit. However, as the offset level of the differential inputs is determined by the difference in the threshold value of the input MIS transistor and since the difference in the threshold value amounts to several tens millivolts it has been impossible to detect a small input.
A differential amplifier utilizing well known MIS transistors will firstly be described with reference to FIG. 1 in which the MIS transistors are of the N channel type. As shown, the drain electrodes of the N channel MIS transistors 11 and 12 are connected to resistors 13 and 14 respectively, the gate electrodes are connected to differential input terminals IN and IN respectively, and the source electodes are commonly connected to a source of constant current 15. This source 15 is not necessarily an ideal one but may be constituted by a MIS transistor having a gate electrode biased by a constant voltage, a grounded source electrode and a drain electrode connected to the source electrodes of the N channel MIS transistors 11 and 12.
While amplified differential outputs of opposite polarity are produced by the drain electrodes of the MIS transistors 11 and 12, when the characteristics of respective component elements are the same, the circuit would be symmetrical with respect to the differential inputs and the input offset would be zero. Actually, however, as there are many unbalances such as the differences in the resistance value of the load resistors 13 and 14, and in the threshold values and the conductances of the MIS transistors 11 and 12 the characteristics of the inverters constituted by said transistors on the left and right sides becomes different. Among the factors that cause asymmetry, the most serious one is the difference in the threshold values of the transistors 11 and 12. For example, when this difference is 50 millivolts, the input offset would be equal to at least 50 millivolts. With the present level of MIS integrated circuit technique, other factors can be reduced to negligibly small value. If one can reduce the input offset to substantially zero regardless of the difference in the threshold values of the transistors, the MIS type differential amplifiers could be used in various applications including various sensors.